Coral Reef

Photo by: Mika Specta

Coral
reefs are not only spectacular marine environments, but they are one of
the oldest ecosystems (community of plants and animals interacting with
their environment) on Earth. They are created by colonies of organisms
called coral polyps (pronounced PAH-lips). Among biological organisms,
only humans have the ability to alter the surface of the planet more than
these tiny marine creatures. Over thousands to millions of years, they may
form massive structures of limestone that alter the shoreline of
continents.

Although coral reefs cover 0.2 percent of the total area of the oceans,
they are critically important for a diversity of marine species. They
provide a habitat for at least 25 percent of all marine animals, including
sponges, more than 4,000 different species of fish, anemones, sea stars,
crabs and other crustaceans, and clams and other mollusks. Home to so many
different species, coral reefs are often referred to as the
"rainforests of the oceans." They also provide physical
barriers to the force of strong waves, protecting beaches, lagoons, and
other coastal features lying behind them. Yet they are highly vulnerable
to pollution, an increase in water temperature, and damage from tourism
and coastal development. By the early twenty-first century, these threats
had already claimed more than one-quarter of the world's coral.

The shape of the land

A coral reef is a wave-resistant limestone structure produced by living
organisms, found principally in shallow, tropical marine waters. Limestone
is a type of rock composed primarily of the mineral calcite, which is a
crystalline form of calcium carbonate (CaCO
3
). Thousands of species inhabit coral reefs, but only a fraction produce
the calcium carbonate that crystallizes into the limestone that forms the
reef.

Coral reefs are one of the oldest ecosystems on the planet. They
provide a habitat for more than 4,000 marine animals and are often
referred to as the "rainforests of the oceans."
PHOTOGRAPH REPRODUCED BY PERMISSION OF

AP/WIDE WORLD PHOTOS

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Coral polyps are the most important reef-building organism. The species
that secrete calcium carbonate are known as hard coral polyps or, simply,
stony corals. In the western Pacific Ocean, more than three hundred
species of stony corals exist. Coral polyps are invertebrates (pronounced
in-VER-tuh-brets), or animals without backbones. They are related to
anemones and jellyfish. In fact, a coral polyp looks similar to an
anemone: it is a jellylike sac attached at one end to its skeleton. The
open end, the mouth, is fringed with six stinging tentacles (or a multiple
of six), which the polyp extends at night to feed. The size of coral
polyps varies greatly, depending on the species. They may be as small as
0.04 inch (0.1 centimeter) in diameter or as large as 8 inches (20
centimeters) in diameter.

Worldwide, coral reefs cover an estimated 110,000 square miles (284,900
square kilometers). Many stony corals grow best in clear, salty water with
a temperature between 70°F and 85°F (21°C and
29°C). Because of this, most reefs are found in the Pacific Ocean,
the Caribbean Sea, the Red Sea, the Arabian Sea, and the Indian Ocean
between the tropic of Cancer and the tropic of Capricorn, two parallel
lines of latitude lying one-quarter of the way from the equator to the
North and South Poles, respectively. Reefs are also found around
Florida and southern Japan because warm-water currents flow into these
areas from the Tropics.

The shape of a coral reef depends on the species of coral building it.
Different types of coral produce different shapes. Some are pointed and
hard; others are round and soft. They may be robust or delicate. Various
shapes in reefs include fingerlike branches, flat branches, knobs or
wedges, boulders or balls, mushroom caps, and tablelike structures.

Coral reef: Words to Know

Atoll:

A ring-shaped collection of coral reefs that nearly or entirely
encloses a lagoon.

Back reef:

The landward side of a reef between the reefcrest and the land.

Barrier reef:

A long, narrow ridge of coral relatively near and parallel to a
shoreline, separated from it by a lagoon.

Coral polyp:

A small, invertebrate marine animal with tentacles that lives within a
hard, cuplike skeleton that it secretes around itself.

Ecosystem:

A system formed by the interaction of a community of plants, animals,
and microorganisms with their environment.

Fore reef:

The seaward edge of a reef that is fairly steep and slopes down to
deeper water.

Fringing reef:

A coral reef formed close to a shoreline.

Invertebrates:

Animals without backbones.

Lagoon:

A quiet, shallow stretch of water separated from the open sea by an
offshore reef or other type of landform.

The process by which plants use energy from sunlight to change water
and carbon dioxide into sugars and starches.

Reef crest:

The high point of a coral reef that is almost always exposed at low
tide.

Symbiosis:

The close, long-term association between two organisms of different
species, which may or may not be beneficial for both organisms.

Zooxanthellae:

Microscopic algae that live symbiotically within the cells of some
marine invertebrates, especially coral.

Most coral reefs are brightly colored, but that color does not come from
the coral polyps themselves. In fact, the bodies of the tiny creatures are
clear. The calcium carbonate they produce to form their external skeleton
is white, much like the bones of a human skeleton. Reef colors come from
the microscopic, single-celled algae called zooxanthellae (pronounced
zoe-ah-zan-THEL-ee) that live in the tissue of the coral. Zooxanthellae
and coral polyps have what is termed a symbiotic (pronounced
sim-bee-AH-tik) relationship. Symbiosis is the close, long-term
association between two organisms of different species, which may or may
not be beneficial for both organisms. In the case of the zooxanthellae and
coral polyps, the relationship benefits both. The algae provide the coral
polyps with more than 90 percent of their nutrients, and the coral polyps
provide
the algae with essential minerals and a protected habitat. Their unique
relationship allows coral reefs to exist and grow year after year.

Coral reefs are generally divided into three classes: fringing reefs,
barrier reefs, and atolls (pronounced A-toles). The simplest and most
common type of coral reef, fringing reefs, form close to the shoreline of
islands and continents. A shallow lagoon, a quiet stretch of water
separated from the open sea, may or may not lie between the shoreline and
the fringing reef. Barrier reefs also form parallel to the shoreline of an
island or a continent, but farther away than fringing reefs. In addition,
they are larger and may stretch for great distances. Wide, deep lagoons
often separate barrier reefs from the shoreline. Atolls are ring-shaped
coral reefs that enclose or nearly enclose a deep lagoon. They are
typically found around islands that have sunk beneath sea level in the
deep ocean. These islands are often the tops of underwater volcanoes. On
the lagoon side of atolls, sediment from broken coral may collect and
partially fill the lagoon, allowing vegetation to grow. Eventually, the
entire lagoon may be filled in, forming an island that may become
inhabited. Atolls such as this are common in the Indian and Pacific
oceans.

Forces and changes: Construction and destruction

Geologists have discovered that ancient corals existed on Earth as long
ago as 400 million years. Present-day stony corals evolved over the last
25 million years. Most established coral reefs are between 5,000 and
10,000 years old. Although they are the largest structures of biological
origin on the planet and represent thousands of years of history, coral
reefs are extremely delicate. Their formation takes place only under
certain conditions.

Corals reproduce two ways. They may do so sexually by releasing eggs and
sperm. Once fertilized, the eggs produce multitudes of free-swimming
larvae (pronounced LAR-vee; immature forms of the coral polyps) that ocean
currents carry great distances. After settling on a suitable hard surface,
the larvae secrete their own calcium carbonate cups and grow into mature
corals, thus forming a new colony and a new reef. Corals may also
reproduce asexually by budding or forming new coral polyps attached to
themselves by thin sheets of tissue and skeletal material. Through
budding, a single coral polyp can develop over time into a massive coral
head.

The hard, cuplike skeleton that a coral secretes around itself from its
lower portion consists of clusters of calcium carbonate, one of the most
common minerals on Earth. In order to produce calcium carbonate, stony
corals need the assistance of zooxanthellae, which live within cells in
the lining of a coral polyp's gut. A polyp not only provides the
zooxanthellae

Like other atolls, Kayangel atoll in the western Pacific Ocean is a
ring-shaped coral reef that encloses a deep lagoon. Atolls are
common in the Indian and Pacific oceans.
PHOTOGRAPH REPRODUCED BY PERMISSION OF

PHOTO RESEARCHERS

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with a protected environment but with carbon dioxide and nutrients that
the polyp gives off as waste products. In return, the zooxanthellae use
the carbon dioxide to provide the coral polyp with nutrients, including
glucose (sugar) and amino acids, through photosynthesis (pronounced
foetoe-SIN-thih-sis). Photosynthesis is the process by which plants use
energy from sunlight to change water and carbon dioxide into sugars and
starches. The polyp uses the compounds supplied by the zooxanthellae to
create proteins, fats, carbohydrates, and calcium carbonate.

Sunlight and clear water

Since zooxanthellae need sunlight in order to photosynthesize, corals
flourish in waters less than 230 feet (70 meters) deep. Maximum growth
rates occur when the coral is in water less than 60 feet (18 meters) deep.
Corals may grow at deeper depths, sometimes up to 300 feet (91 meters),
but their ability to produce calcium carbonate is greatly reduced. As a
result, they grow poorly. The growth rate of corals is also reduced if the
water is murky and sunlight is not able to penetrate. Water temperature
and the amount of salt in the water also affect growth rate. Corals may
grow in water slightly below 70°F (21°C) and slightly above
85°F (29°C), but that growth is very slow. Stony corals also
prefer waters where the salt concentration ranges between 32 and 42 parts
per thousand. The concentration of oxygen in the water must also remain
high.

When optimum conditions are present, corals may grow as much as 1.7 inches
(4.5 centimeters) per year. Different species grow at different rates. In
general, the larger the coral, the slower the growth. Very large corals
may grow only 0.2 to 0.8 inch (0.5 to 2 centimeters) per year. Branching
corals may grow much faster, up to 8 inches (20 centimeters) per year.
Because of their fast growth rate and their treelike structure, branching
corals are prone to damage from strong storm waves, which may break off
their branching limbs. Large, dense corals are much more stable and less
prone to damage from wave action.

While a coral polyp is alive, it periodically rises up in its cuplike
skeleton and secretes more calcium carbonate to form a new base or floor.
In this way, the polyp enlarges its skeleton, creating chambers underneath
its base. When a polyp dies, its skeleton remains, becoming the foundation
on which a new polyp attaches and builds its skeleton. Coral reefs are
composed of layer upon layer of polyp skeletons, sometimes numbering in
the billions. Only the thin, top layer contains living coral polyps.

Other creatures add to the complex structure of a coral reef. A type of
red algae known as coralline algae contribute to the framework of reefs by
secreting their own encrusting skeleton that helps cement loose sediment
on the reef. Other organisms that contribute reef sediments include
sponges, clams, and snails. These marine creatures live in the holes and
crevices of the reef. When they die, their remains provide a foundation
for new coral polyps.

Reef structure and forms

Coral reefs remain underwater except when water levels periodically
decrease, such as at low tides. At this time, only the highest part of the
coral reef, the reef crest, is exposed, washed over by waves. Coral polyps
that exist and grow on this section of a reef must be able to tolerate the
Sun's heat and wave action. In order to do so, they generally grow
in plate-like, stubby branching or massive structures. The upper surface
is often encrusted with coralline algae.

Corals that exist on the reef seaward from the reef crest are diverse and
show the greatest range of forms. In areas where wave energy is still
high, massive corals are predominant. As the reef angles deeper and deeper
beneath the water's surface, delicate branching corals take over.
The outermost seaward slope is called the fore reef. It may angle down to
the sea bottom by as much as 30 degrees. The fore reef consists of
limestone boulders, coral branches, and smaller sediments. It may have
deep channels cut into it, forming fingerlike structures that extend
seaward from the reef. These help stabilize the reef and cut down the
force of incoming strong waves.

Charles Darwin.
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Darwin and Atoll Formation

While traveling aboard the
H.M.S. Beagle
in the mid-1830s, English naturalist Charles Darwin (1809–1882)
devised the modern theory of coral atoll formation. Although widely
known for his theory of evolution by natural selection, Darwin made many
contributions to the science of geology. His theory of coral reefs is
his best-known.

Darwin proposed that fringing reefs, barrier reefs, and atolls
represented a series through geologic time. He held that the transition
from fringing reef to barrier reef to atoll could result from the upward
growth of coral on the edge of a gradually sinking volcano. He believed
that barrier reefs represented a middle stage between fringing reefs and
atolls, and that the ringlike appearance of an atoll with a central
lagoon resulted from the total submergence of the summit of a volcano.

In the 1950s, scientists from the U.S. Geological Survey, an earth
science research and information agency, undertook extensive drilling
programs on atolls in the Pacific Ocean. Hundreds of feet down, they
encountered volcanic rock, proving Darwin's theory that atolls
are perched over ancient submerged volcanoes.

The area lying landward of the reef crest is known as the back reef. Sand
and other fine-grained sediment often inhibit reef growth in this area, so
various other marine organisms dominate. However, scattered stubby,
branching, or low knobby corals may develop in water as shallow as 3 feet
(1 meter). Beyond the back reef, the water begins to deepen again, to as
much as 100 feet (30 meters) or more, within the lagoon. Protected from
the full force of waves, the lagoon floor is smooth and fine-grained.

While fringing and barrier reefs are common on the submerged portions of
continents, atolls are not. Atolls formed in the deep ocean around
submerged submarine volcanoes. They began to grow as a small fringing reef
around the shoreline of a volcanic island. Once the volcano had become
extinct, the sea floor beneath it may have begun to subside or sink under
the weight of the volcano. As the island slowly submerged over millions of
years, the corals continued to grow upward to the surface of the water,
keeping pace with the rate the island was sinking. The sides of some
atolls reach depths as great as 1,500 feet (457 meters). Eventually, as
the island slipped beneath the ocean's surface, a ring of coral
reefs remained, surrounding a central lagoon.

Cold-water Corals

Most well-known corals exist in tropical coral reefs, but reef-forming
corals also exist in deep, cold water. Known as
Lophelia,
these corals lack zooxanthellae, so they do not depend on sunlight for
survival. Instead, they feed by capturing food particles with their
tentacles from the surrounding water. They are found at depths ranging
from 230 to 3,280 feet (70 to 1,000 meters). These types of corals grow
slower than their warm-water relatives, averaging 0.04 inch (0.1
centimeter) per year.

The largest known
Lophelia
coral reef is found off central Norway at a depth of about 1,312 feet
(400 meters). It measures almost 9 miles (14 kilometers) in length and 2
miles (3 kilometers) in width. It stands almost 98 feet (30 meters) in
height.

Cold-water corals are found throughout the western Atlantic Ocean from
Nova Scotia to Brazil. They are also found in the eastern Atlantic, the
Mediterranean Sea, the Indian Ocean, and eastern Pacific Ocean. Like
warm-water reefs,
Lophelia
coral reefs support very rich communities of fish, shrimps, and other
invertebrates.

Some atolls formed when sea levels rose, submerging the tops of islands.
The rise and fall of the sea level over the past few million years has
been caused by changes in the volume of water tied up in land glaciers and
ice sheets during the ice ages. When ice sheets grew in the Northern
Hemisphere, the sea level dropped and coral reefs such as atolls were
stranded above the waterline. Since present-day water levels have not
risen to what they were before the last ice age, which ended approximately
10,000 years ago, the tops of many atolls have remained exposed.

Reef damage

While coral reefs may be damaged by natural forces such as storms and
hurricanes, they suffer the severest damage from human activity. Reefs are
often destroyed by collectors who use coral to create jewelry and by
fisherman who use poison or dynamite to catch fish around coral reefs.
Because corals need sunlight and sediment-free water to survive, water
pollution poses a grave danger. Oil spills, the dumping of sewage wastes,
and the runoff of soil and agricultural chemicals such as pesticides all
threaten the delicately balanced ecosystem of coral reefs.

Global warming, an increase in the world's temperatures, is the
biggest threat facing coral reefs. It is thought to be caused, in part, by
the burning of fossil fuels and the depletion of the ozone layer, both
brought about through human activities. Scientists believe a warming of
water temperature, by even just a few degrees, can cause coral polyps to
expel the zooxanthellae living inside them. This results in coral
bleaching, which is the whitening of coral colonies due to the loss of the
zooxanthellae. The end result is death of the coral. While pollution and
changes in the salt content of the water can also bring about coral
bleaching, warmer sea temperatures seem to be the biggest culprit. Natural
occurrences, such as El Niño (the irregular periods during which
the normally cold waters off the coast of Peru are made warmer by the
arrival of warm waters from the equatorial region), have been blamed for
some coral bleaching. However, these events are short-lived. Global
warming is not.

Spotlight on famous forms

Great Barrier Reef, off the northeastern coast of Australia

The Great Barrier Reef, situated off Queensland state in
Australia's northeast, is the largest structure on the planet
created by living organisms. Approximately 1,250 miles (2,011 kilometers)
in length, the coral reef consists of more than 2,800 individual detached
reefs, separated by deep channels. It is separated from the Australian
shoreline by a shallow lagoon that varies from 10 to 100 miles (16 to 161
kilometers) in width. At its widest, the reef measures 45 miles (72
kilometers) across. It covers roughly 80,000 square miles (207,200 square
kilometers), an area approximately as large as the state of Kansas. The
reef can be seen from space and was first mapped by
Apollo 7
astronauts in 1968.

Geological evidence shows the reef began growing more than 25 million
years ago. Its age and size are due to the very stable geological setting
of the area off the Australian continent and the favorable circulation of
oceanic water. Winds in the area help stir the water, keeping it a
relatively constant warm temperature regardless of depth.

The Great Barrier Reef covers roughly 80,000 square miles, an area
approximately as large as the state of Kansas. It is the largest
structure created by living organisms.
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CORBIS CORPORATION

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The reef is home to millions of living creatures. More than 350 species of
stony corals make up the reef. Countless other organisms inhabit the area,
including more than 1,500 species of fish, 4,000 species of mollusks, 200
species of birds, as well as varieties of dolphins, whales, green turtles,
and dugongs or "seacows."

The Great Barrier Reef is a major tourist destination, attracting more
than 2 million tourists each year. To prevent the reef's
destruction, the Australian government established the Great Barrier
Marine Park in 1975. Encompassing most of the reef, the park is the
world's largest protected marine area. Despite this protection, the
reef is still endangered by rising sea temperatures and human activity. In
2003, the Australian government sought to ban commercial and recreational
fishing from more than 30 percent of the reef.

Kiritimati, Pacific Ocean

With a total area of 222 square miles (575 square kilometers), Kiritimati
is the largest coral atoll in the world. It encloses a large lagoon, which
accounts for almost half of its area, and more than 100 lakes or ponds.
Lying 145 miles (233 kilometers) north of the equator, it is part of
the Republic of Kiribati, composed of 33 islands scattered across 2,400
miles (3,860 kilometers) of the Pacific Ocean. In the local language,
"ti" is pronounced "s," so Kiritimati is
pronounced ki-RIS-mas.

English explorer and navigator James Cook (1728–1779) landed on the
atoll in 1777. Because the day of his arrival was Christmas Eve, Cook
named the atoll Christmas Island (not to be confused with the
Australian-administered Christmas Island that lies in the eastern Indian
Ocean south of Java). More than a century later, England claimed the atoll
as part of the Gilbert and Ellice Islands colony, extending its rule over
it. In the late 1950s and early 1960s, England and the United States
conducted nuclear tests on the atoll. In 1979, the Republic of Kiritabi
was granted full independence.

Despite the fact that some areas of Kiritimati still remain barren as a
result of nuclear tests, the atoll boasts much wildlife. It is
particularly important as a seabird-nesting site: an estimated six million
birds use or breed on the atoll. In 1975, Kiritimati was declared a
wildlife sanctuary.

Maldives, Indian Ocean

The Maldives is an archipelago (pronounced ar-keh-PELL-ah-go; a group or
chain of islands) of almost 1,200 coral islands in the Indian Ocean,
located about 420 miles (675 kilometers) southwest of Sri Lanka. The
archipelago is 511 miles (823 kilometers) long and 81 miles (130
kilometers) at its greatest width. The total area including land and sea
is approximately 34,750 square miles (90,000 square kilometers). About 2
percent of this is land.

Many of the islands are small, level, and low-lying, often no more than
6.5 feet (2 meters) above sea level. Some are gradually washing away into
the ocean, while others are in the process of formation and are constantly
growing in size. The island of Malé, location of the capital city,
is the most densely populated and developed. It is 1.2 miles (2
kilometers) long and just over 0.5 mile (0.8 kilometer) wide.

The islands are formed from the growth of coral over a long-submerged
volcanic mountain range. A protective fringing coral reef surrounds each
island, some of which have freshwater lagoons. These are true coral
islands: there is no trace of yellow or black coloring in the sandy coral
beaches, as there is on other beaches around the world. Because the soil
on the islands is completely coral-based, it is poor in nutrients and
thick jungles do not grow. The coconut palm is the most common tree, and
it grows densely on many of the islands. There are no hills, mountains, or
rivers on any of the islands.

The Maldives is an archipelago of almost 1,200 coral islands in the
Indian Ocean. The islands are formed from the growth of coral over a
long-submerged volcanic mountain range.
PHOTOGRAPH REPRODUCED BY PERMISSION OF THE

I would recommend gnitteg really hardy fish to start off the nitrogen cycle in the aquarium. I don't really know what to tell you when it comes to corals and all but I know a ton about the fish. The fish can get really expensive so look forward to that. But when you add the first fish, you should add some damsels or chromis because they are really hardy but keep in mind that damsels are really aggressive towards their own species, so get at least 3 or 4. They are cheap too. And any type of clownfish are hardy, but you need to either get one or a lot because they are very aggressive towards their own species too. So buy a pair or just one because it will be hard to add one later on if you just get one. And also I wouldn't recommend mixing species either. Keep in mind if you ever want any angel fish, make sure your tank is established because they are very delicate and can get stressed and die very easily. You should do some research or ask your local aquarium questions about fish. One last thing, if you are ever going to get any Tangs like Dory or whatever. They can get territorial and watch the shape of the body. If Tangs have the same body shape, they will fight, but different body types won't so look out for that but you may get lucky, you never know. I hope this helped and good luck! Was this answer helpful?